Hydrostatic or “hystat” drive refers generally to a drive train or a portion of a drive train in a work machine that utilizes hydraulic fluid pressurized by engine rotation as the motive force for propelling the work machine. In a typical design, a pump is driven with an output shaft of the engine and provides pressurized hydraulic fluid to a hydraulic motor, in turn coupled with one or more axles of the work machine. Commonly, both the pump and the motor have a variable displacement, allowing the relative torque and speed applied to a drive shaft of the work machine, and in turn to the wheels or tracks thereof to be varied.
For example, where a work machine operator wishes to provide a relatively high torque to the work machine wheels or tracks, the displacement of the motor will be relatively large such that, at a given hydraulic pressure from the pump, a relatively large force is transferred to the wheels or tracks for each stroke of the motor. Where a relatively lower torque is desired, for example when operating the work machine at a relatively higher velocity, the relative displacement of the motor can be decreased, and its relative stroking speed increased by increasing the pump displacement.
While the combination of a variable displacement pump and variable displacement motor in a hystat drive work machine creates tremendous flexibility in operation, there is room for improvement. In many known designs, the efficiency and smoothness of various operations in the hystat drive system is limited by the physical capabilities of the work machine operator, as well as the limitations of the various system components. In particular, slowing and accelerating the work machine can be relatively rough or inefficient if the operator is not well practiced in adjusting the pump and motor.
Where an operator unwittingly adjusts a pump or motor displacement too quickly, the relatively rapid change in torque provided by the motor to the ground engaging wheels or tracks can be problematic. Excessively high torque, or changes in torque can induce in the work machine an excessively large acceleration or deceleration, or increase or decrease in the same, known in the art as “jerk.” Operation of the work machine may thus not only be uncomfortable for the operator but can also risk tipping the machine or spilling materials loaded thereon. Conversely, where an operator adjusts the motor or pump too slowly, he or she risks stalling the work machine, or at least slowing the performance of various tasks unnecessarily.
Highly skilled operators can typically execute various work machine operations relatively rapidly and smoothly. However, many modern hystat systems simply have too many variable components for a single operator to optimally control or monitor a changing pump displacement, motor displacement, throttle position, and other functions. Designers have developed various systems wherein an electronic controller monitors and adjusts one or more of the parts of the hydrostatic drive. While these systems have shown some improvements over earlier designs, the operator is still typically responsible for controlling certain of the components, such that smooth operation and efficiency can still be compromised in many situations.
Co-owned U.S. Pat. No. 5,624,339 shows a method for controlling shift points in a continuously variable transmission that includes a hydrostatic drive path or a combined hydrostatic and mechanical transmission drive path. The mechanical transmission includes a planetary summing arrangement that appears to allow for smooth shift without disruption of torque. Although this strategy and structure appears promising, there always remains room for improving upon the overall combination of work efficiency with rider comfort.
The present disclosure is directed to one or more of the problems or shortcomings set forth above.